1. Field of the Invention
This invention relates to an overhead projector sheet (hereinafter simply referred to as "OHP" sheet) which is printable by thermal transfer printing, in which an image of desired printing quality can be obtained using a thermal transfer printer, and to a method for thermal transfer printing on the OHP sheet.
2. Prior Art Statement
Various printers have been put to practical use, such as impact type printers, typically wire dot type printers, and nonimpact type printers, typically ink jet type or thermal transfer type printers.
Further, many kinds of thermal transfer printers have also been put to practical use and OHP sheets for thermal transfer printing have also been proposed.
For example, there has been proposed an OHP sheet comprising a transparent film applied thereon with a rubber based adhesive layer, an acryl based adhesive layer or a polyester based adhesive layer. There has also been proposed an OHP sheet provided with a coating layer including a solid wax having a good compatibility with a thermal fusible ink (Japanese Patent Public Disclosure Sho 60(1985)-154096).
There are two types of thermal transfer printers, the serial type and the line type. The line printer is superior to the serial printer in terms of printing speed.
For obtaining color images using a line type color thermal transfer printer, there have been developed drum, rocking and multihead printers which use an ink ribbon applied with ink materials of various colors along its longitudinal direction. In this case, with the exception of the multihead printers, the material to be printed has to be reciprocally moved three to four times between a head and a platen or drum in order to print, for example, three to four colors of ink onto the material to be printed.
This printing system will now be described with reference to a color thermal transfer printer of drum type as shown in FIG. 1. A material 6 to be printed is wound around a rotary drum 1 such that one end of the material 6 is retained by a clamp portion la, whereas an elongated ink ribbon 5 applied, in turn, with ink materials of various colors along its longitudinal direction is contacted with the outer peripheral portion 1b of the rotary drum 1 from a feed roller 2 and the leading end thereof is taken up by a take-up roller 3.
During printing, the rotary drum 1 is rotated in the direction E (counterclockwise) and the take-up roller 3 is rotated to take up the ink ribbon 5 so that the ink material for the first color will be brought to the outer peripheral portion 1b of the rotary drum 1, and a thermal head 4 is moved toward the material 6 to be printed on the outer periphery 1b of the rotary drum 1 in such a manner as to press onto the ink ribbon 5. By this, the ink material of the first color on the ink ribbon 5 is heated, melted and transferred to the material 6 to be printed. When one color has been printed on one image plane, the rotary drum 1 is rotated in the direction F (clockwise), i.e., the opposite direction to that during printing, so as to feed the material 6 to be printed reversely, or the rotary drum 1 is rotated further in the direction E to feed the material 6 normally so that the leading edge of the image plane of the material 6 is returned to the, position where the thermal head 4 is located, the ink ribbon 5 is taken up in such a manner as to bring the ink material for the second color to the outer periphery of the drum, the ink material is pressed against and transferred to the material 6 to be printed by the thermal head 4, and this procedure is repeated as many times as the number of colors.
FIG. 2 shows another example of the drum type, color thermal transfer printer. In this case, the material 6 to be printed is retained at its one end by the clamp portion la of the rotary drum 1 via the platen 7. On the other hand, the ink ribbon 5 is urged against the material 6 on the platen 7 by the movement of the thermal head 4. By this, the first color ink material of the ink ribbon 5 is melted and transferred to the material 6 to be printed.
When the first color has been printed by the thermal head 4, the rotary drum 1 is rotated in the opposite direction to the direction toward which the rotary drum 1 was rotated during printing. As a result, the material 6 is reversely moved on the platen 7 so that the leading edge of the image plane is returned to the position where the thermal head 4 is located, thereby to print the next color.
More specifically, where a single color image is to be obtained with a line thermal transfer printer, or where a multicolor image is to be obtained with a multihead thermal transfer printer, the material to be printed is required to be passed between the thermal head and the platen or rotary drum only once. On the other hand, where a multicolor image is to be obtained using a color thermal transfer printer of drum type or rocking type, the material to be printed is passed between the thermal head and the platen or rotary drum as many times as the number of colors
The material to be printed is reciprocally moved by a belt, a roller, etc. within the printer. During the reciprocal movement of the material to be printed, the printing surface and the rear surface of the material to be printed are in contact with the belt, the roller, etc.
In the event the material to be printed is a sheet of paper, the above-mentioned contact does not create serious problems. However, in the event the material to be printed is an OHP sheet comprising a transparent film, it often incurs feed scars on its rear surface during reciprocal movement. The result is that when it is projected by an overhead or other type projector, the scarred portions appear as black lines when projected. Thus, the image becomes difficult to see.
If the OHP sheet is reciprocally moved between the thermal head and the platen or rotary drum as mentioned above, a displacement of the dots occurs. The result is that the obtained image is undesirably blurred.
A further disadvantage occurs because heat does not necessarily propagate uniformly through the OHP sheet, particularly at its coated layer on the transparent film. The result is that when an ink material is transferred by a thermal head, etc., an uneven color thickness occurs and, therefore, good quality printing is difficult to obtain.
An additional disadvantage occur because of the fact that since the OHP sheet itself is transparent, if several sheets are placed one upon another, such sheets become difficult to distinguish individually due to reflected light.